The transport of neutral amino acids in mammalian cells is broadly described as occurring through two parallel pathways, one Na+-dependent and the other Na+-independent. The "Na+-gradient hypothesis" proposes that the electrochemical potential gradient for Na+ serves as the sole energy source for driving accumulation of amino acids by the Na+-dependent system. The source of energy for uptake of Na+-independent amino acids is not known. This research is based on two hypotheses: (1)\the cell membrane potential plays an essential role in the provision of nutrients necessary for cellular metabolism and (2)\a single transport pathway serves for the cellular uptake of both Na+-dependent and Na+-independent neutral aminoacids. We have determined in the Ehrlich ascites tumor cell: (1)\the coupling stoichiometry of Na+-amino acid co-transport is consistent with the transport of one Na+/amino acid; (2)\the Na+ electrochemical potential gradient is sufficient to provide the energy necessary for amino acid accumulation; (3)\the cell membrane is repolarized by active Na+ extrusion following addition of amino acids; and (4)\H+ may substitute for Na+ as a co-substrate suggesting that the same transport system serves for Na+-dependent and Na+-independent amino acids. Ongoing studies are directed toward the evaluation of proton gradients as sources of energy for Na+-independent amino acid transport and the interactions of Na+-\and H+-dependent co-transport. We anticipate that this investigation will lead to a more complete understanding of the energetics involved in meeting the cell's nutritional requirements. (A)